As circuit dimensions continue to decrease, it has become increasingly important to provide ways to remove heat from the circuit die. Indeed, a significant limiting factor in the speed and density of electronic devices is the ability to satisfactorily remove the heat that such devices develop while operating. To this end, heat spreaders are now commonly employed in many circuit packages.
FIG. 1 illustrates an encapsulation step in the prior art for a Fully Molded and Separated Ball Grid Array (FSBGA). A partially formed FSBGA device 10 includes a circuit die 20, a spacer 30 and a substrate 40 adhered to each other. The circuit die 20 may be any die known in the art, such as a silicon substrate, a GaAs substrate, a silicon-on-glass substrate, etc. The spacer 30 may also be made of any of these substrate materials, which may be chosen for high thermal conductivity characteristics, and which is typically made of silicon. A first adhesive layer 12 adheres the backside of the spacer 30 to an active surface 22 of the circuit die 20. The spacer 30 is basically a non-active die, and primarily serves to transfer heat away from the surface of the circuit die 20. The spacer 30 thus typically has no electrical connection to any external circuitry. A second adhesive layer 14 adheres a backside 25 of the circuit die 20 to the substrate 40. The second adhesive layer 14 may be either electrically conductive or non-conductive, but is usually conductive to provide greater thermal dissipation properties. The substrate 40 is typically a laminate, and includes non-conductive regions 42 and conductive regions 44. The non-conductive regions 42 may be made from, for example, an organic material, such as Bismaleimide Triazine (BT), and the conductive regions 44 may be made from copper, aluminum or the like. Vias 46, filled with a conductive material, provide a conductive pathway that electrically connects conductive regions 44 on the top surface 41 with their respective counterpart regions on the bottom surface 49 of the substrate 40. Bond wires 50 electrically connect pads 24 on the active surface 22 of the circuit die 20 with corresponding wedges 48 on the top surface 41 of the substrate 40. The wedges 48 electrically connect with the vias 46, and hence with the bottom surface 49 of the substrate 40, by way of the conductive regions 44.
The partially formed FSBGA device 10 is disposed within a mold 60 to undergo an encapsulation process. Although only a single device 10 is shown in FIG. 1, it will be appreciated that typically multiple devices 10 are encapsulated at once within the mold 60. The mold 60 includes a top plate 62 and a bottom plate 64; a cavity 66 between the top plate 62 and bottom plate 64 is filled with a molding material 70, indicated in FIG. 2. However, to insure that a maximum amount of heat can escape from the circuit die 20, it is highly desirable that the top surface 32 of the spacer 30 remain exposed. That is, it is desired that no molding material 70 cover the top surface 32 during the encapsulation process.
The encapsulation process is analogous to injection molding procedures used to make, for example, plastic goods. A considerable amount of pressure may be exerted upon the molding material 70 within the cavity 66, and as a result, even small gaps between the top surface 32 of the spacer 30 and the top mold plate 62 can lead to mold flash over the top surface 32. Hence, it is essential that the top surface 32 be flush against the top mold plate 62 during the encapsulation process.
To further complicate matters, because of the pressures involved, a considerable amount of force is exerted between the top mold plate 62 and the bottom mold plate 64. Extreme care must be taken, then, to precisely control the thicknesses of the circuit die 20, spacer 30 and the adhesive layers 12, 14. If the device 10 is too thick, pressure exerted by the mold 60 upon the spacer 30 can cause the relatively fragile circuit die 20, and even the spacer 30, to break. On the other hand, if the device 10 is too thin, mold flash will form on the top surface 32 of the spacer 30, severely degrading the heat dissipating characteristics of the device 10. To provide for greater tolerances during the encapsulation process, then, a thin film 68 may be disposed over the inside surface of the top mold plate 62. The film 68 may serve both as a cushioning layer for the electrical device 10, and as a sealing layer to prevent mold flash. Because the molding process is single-sided, the bottom surface 49 does not develop any mold flash. Molding material 70 only flows over the top surface 41 of the substrate 40.
After the encapsulation process, molding material 70 fills the cavity 66, as shown in FIG. 2. A top surface 72 of the molding material 70 lies flush with the top surface 32 of the spacer 30, which, uncovered by any molding material 70, remains exposed to maximize thermal dissipation. Thereafter, the molding material 70 undergoes a curing process to harden the molding material 70. This is typically a heat curing process, which is performed in an oven. A solder ball mounting process is then performed to dispose a plurality of solder balls 80 onto respective conductive regions 44 of the bottom surface 49 of the substrate 40, as shown in FIG. 3. A singulation step separates the various FSBGA devices from each other to provide individual FSBGA packages, and then a heat spreader attachment step is performed.
As shown in FIG. 4, the heat spreader attachment step begins by laying down an adhesive layer 16 over the top surface 72 of the molding material 70 and the top surface 32 of the spacer 30. The adhesive 16 may be selected for superior thermal conductivity characteristics. As shown in FIG. 5, a heat spreader 90 is then attached to the adhesive layer 16. The heat spreader 90 is preferably made from a highly heat-conductive material, such as copper, and may be further provided fins or like protrusions to maximize its surface area. Because the heat spreader 90 is almost directly in contact with the top surface 32 of the spacer 30, but for the relatively thin adhesive layer 16, the FSBGA device 10 exhibits superior thermal dissipation characteristics. The adhesive layer 16 then undergoes a curing process, which is typically a thermal process performed in an oven, to secure the heat spreader 90 to the top surfaces 72, 32, and then a final laser marking step is performed to complete the FSBGA device 10.
The prior art encapsulation process requires that extremely tight tolerances be maintained on the thicknesses of the circuit die 20, spacer 30 and the adhesive layers 12, 14. Die thickness tolerances are typically between ±12.5 μm, as are those for the adhesive layers. This remains true even when the thin film 68 is used, since the film 68 may not provide a sufficient cushioning effect to prevent die cracking. Additionally, encapsulation with the thin film 68 is a more expensive procedure, which leads to higher production costs. Accordingly, there is an immediate need for an improved encapsulation process for circuit dies.